We applied a method recently developed in this laboratory--"fracture-label"--to investigate the topology of lectin (concanavalin A and wheat germ agglutinin) binding sites with plasma membrane halves of cancer vs. normal cells. We used ovary-dependent breast carcinomas of the rat (DMBA and MTW9). Cells from growing and regressing tumors were studied and compared with epithelial cells from normal rat mammary gland. The aim of the project was to search for differences in the partition, on freeze-fracture, of membrane glyconjugates of mammary gland cells after malignant transformation, and during tumor regression. In all cases the lectin receptors were preferentially seen in association with the exoplasmic halves of the plasma membrane; significant differences in Con A and WGA labeling among normal, malignant, and tumor regressing cells were not disclosed.